Quantum optics laboratory

What is light? How big is a photon? Is a photon indivisible? The questions are numerous, but the answers are more controversial and problematic than many people are willing to admit. In our research group we try to look into details that are usually treated only superficially, at least in the experimental context: the details regarding the photon creation, the photon propagation in time and space, its interaction with materials and the absorption process. We hope to contribute to new insights into the nature of light.

A beam splitter is an essential component in almost any quantum-optical experimental setting. Most often it is described as a simple device that through a simple flip-coin process decides whether a photon should pass straight through, or be reflected. But could it be that the processes involved are more complex? If so, could it be that some of the conclusions that hitherto have been drawn may be incorrect?

About the group

During the last, few years we have built a modern quantum optics laboratory at the Department of Physics, where the nature of light is in focus.

The group is so far very small. The groups is expanding very slowly, as it takes time to raise money for equipment. The present situation is quite satisfactory: today the staff consists of one associate professor, one PhD-student and one post-doc (new). Also, several graduate students have expressed their interest in doing their master thesis at the group. The activity is also extended through a discussion group that meets every second Thursday.

Projects

1) Entanglement

Entangled photons have played a crucial role on deciding “whether Bohr or Einstein was right” in their discussion in the 1930s. Can we say that light in fact has a definitive polarization when it travels in time and space? If so, the light is described in a “local realistic” way. Or is it so that the only thing we can say is that the light has a possibility to show a polarization in a given direction only if we carry out a measurement? Einstein preferred local realism, Bohr meant that was inacceptable.

Aspect’s experiments in Paris in 1981, and numerous experiments since, are most often taken as a support for Bohr’s view.

However, the result is quite remarkable and creates serious problems. Could it be that there are some faster than light communication between two entangled photons, implying that Einstein’s theory of relativity is wrong? If not, the implication is that the “cause-effect concept” of causality in philosophy may be rejected in some way or another (a "cause" working on one particle somewhere in the universe leads to an immediate "effect" at a quite distant place). In our opinion we cannot reject such an important concept before we have seriously tried to find other interpretations of our observations.

We want to carry out experiments with entangled photons and track the assumptions in the analysis more carefully than it is usually done. We wish to model light as a dividable wave, and we assume that the detectors are not working according to the principle "one photon in - one pulse out". We will record every “click” from the detectors with a time stamp unit, and analyze the results after the data gathering is completed. Normally, most of the data is neglected - only the data that fits the “indivisible photon” model are kept. Thus there is a risk of introducing a serious bias.

2) Coherence / size of a photon

In our group one hypothesis under consideration is that a photon is spread out to some extent in time and space. It is not an almost point-like particle. If the hypothesis reflects the reality, how big is then a photon? Are photons from different sources of different sizes? Could it be so that the size of the photon in some way or another is tied to the coherence length of the light? If so, coherence length measurements may be a valuable tool in order to explore properties of the photon.

We have started with numerical modeling of how coherence length depends on the size of a photon and also with the intensity of the light. Our results are interesting and promising. We now want to carry out measurements of coherence lengths for light from various sources, when the intensity is reduced to nearly the so-called one-photon level. The intensity may be reduced by attenuating the light by neutral density filters, or it may be reduced by utilizing fewer emitters in the source. If the result comes out differently for those two experimental situations, it may indicate that photons are not as indivisible as many scientists today think. We look very much forward to see what the experiments will tell us.